This description relates to multiple-pass interferometry.
Displacement measuring interferometers monitor changes in the position of a measurement object relative to a reference object based on an optical interference signal. The interferometer generates the optical interference signal by overlapping and interfering a measurement beam reflected from the measurement object with a reference beam reflected from the reference object.
Referring to FIG. 1, a typical interferometry system 10 includes a source 20, an interferometer 30, a detector 40, and an analyzer 50. Source 20 includes a laser for providing an input beam 25 to interferometer 30. In one example where heterodyne interferometry technique is used, input beam 25 includes two different frequency components having orthogonal polarizations. An acousto-optical modulator may be used to introduce frequency splitting to produce the two frequency components. Alternatively, source 25 may include a Zeeman-split laser to produce the frequency splitting. In another example, where homodyne interferometry technique is used, input beam 25 may have a single wavelength.
In a heterodyne interferometry system, the orthogonally polarized components are sent to interferometer 30, where they are separated into measurement and reference beams. The reference beam travels along a reference path. The measurement beam travels along a measurement path. The reference and measurement beams are later combined to form an overlapping pair of exit beams 35. The interference between the overlapping pair of exit beams contains information about the relative difference in optical path length between the reference and measurement paths. In a homodyne interferometry system, a non-polarizing beam splitter may be used to separate the input beam into the measurement and reference beams.
In one example, the reference path is fixed and the changes in the optical path length difference correspond to changes in the optical path length of the measurement path. In another example, the optical path length of both the reference and measurement paths may change, e.g., the reference path may contact a reference object that may move relative to interferometer 30. In this case, changes in the optical path length difference correspond to changes in the position of the measurement object relative to the reference object.
When the reference and measurement beams have orthogonal polarizations, the intensity of at least one intermediate polarization of the overlapping pair of exit beams is selected to produce the optical interference. For example, a polarizer may be positioned within interferometer 30 to mix the polarizations of the overlapping pair of exit beams, which is then sent to detector 40. Alternatively, the polarizer may be positioned within detector 40.
Detector 40 measures the intensity of the selected polarization of the overlapping pair of exit beams to produce the interference signal. Detector 40 includes a photodetector that measures the intensity of the selected polarization of the overlapping pair of exit beams. Detector 40 may also include electronic components (e.g., an amplifier and an analog-to-digital converter) that amplifies the output from the photodetector and produces a digital signal corresponding to the optical interference.
In many applications, the measurement and reference beams have orthogonal polarizations and different frequencies. The different frequencies can be produced, for example, by laser Zeeman splitting, by acousto-optical modulation, or internal to the laser using birefringent elements or the like. The orthogonal polarizations allow a polarizing beam splitter to direct the measurement and reference beams to the measurement and reference objects, respectively, and combine the reflected measurement and reference beams to form overlapping exit measurement and reference beams. The overlapping exit beams form an output beam that subsequently passes through a polarizer.
The polarizer mixes polarizations of the exit measurement and reference beams to form a mixed beam. Components of the exit measurement and reference beams in the mixed beam interfere with one another so that the intensity of the mixed beam varies with the relative phase of the exit measurement and reference beams. A detector measures the time-dependent intensity of the mixed beam and generates an electrical interference signal proportional to that intensity. Because the measurement and reference beams have different frequencies, the electrical interference signal includes a xe2x80x9cheterodynexe2x80x9d signal having a beat frequency equal to the difference between the frequencies of the exit measurement and reference beams.
If the lengths of the measurement and reference paths are changing relative to one another, e.g., by translating a stage that includes the measurement object, the measured beat frequency includes a Doppler shift equal to 2vnp/xcex, where v is the relative speed of the measurement and reference objects, xcex is the wavelength of the measurement and reference beams, n is the refractive index of the medium through which the light beams travel, e.g., air or vacuum, and p is the number of passes to the reference and measurement objects. Changes in the relative position of the measurement object correspond to changes in the phase of the measured interference signal, with a 2xcfx80 phase change substantially equal to a distance change L of xcex/(np), where L is a round-trip distance change, e.g., the change in distance to and from a stage that includes the measurement object.
Unfortunately, this equality is not always exact. In addition, the amplitude of the measured interference signal may be variable. A variable amplitude may subsequently reduce the accuracy of measured phase changes. Many interferometers include non-linearities such as what are known as xe2x80x9ccyclic errors.xe2x80x9d The cyclic errors can be expressed as contributions to the phase and/or the intensity of the measured interference signal and have a sinusoidal dependence on the change in optical path length pnL. In particular, the first harmonic cyclic error in phase has a sinusoidal dependence on (2xcfx80pnL)/xcex and the second harmonic cyclic error in phase has a sinusoidal dependence on 2 (2xcfx80pnL)/xcex. Higher harmonic cyclic errors can also be present.
There are also xe2x80x9cnon-cyclic non-linearitiesxe2x80x9d such as those caused by a change in lateral displacement (i.e., xe2x80x9cbeam shearxe2x80x9d) between the reference and measurement beam components of an output beam of an interferometer when the wavefronts of the reference and measurement beam components have wavefront errors. This can be explained as follows.
Inhomogeneities in the interferometer optics may cause wavefront errors in the reference and measurement beams. When the reference and measurement beams propagate collinearly with one another through such inhomogeneities, the resulting wavefront errors are identical and their contributions to the interferometric signal cancel each other. More typically, however, the reference and measurement beam components of the output beam are laterally displaced from one another, i.e., they have a relative beam shear. Such beam shear causes the wavefront errors to contribute an error to the interferometric signal derived from the output beam.
Moreover, in many interferometry systems beam shear changes as the position or angular orientation of the measurement object changes. For example, a change in relative beam shear can be introduced by a change in the angular orientation of a plane mirror measurement object. Accordingly, a change in the angular orientation of the measurement object produces a corresponding error in the interferometric signal.
The effect of the beam shear and wavefront errors will depend upon procedures used to mix components of the output beam with respect to component polarization states and to detect the mixed output beam to generate an electrical interference signal. The mixed output beam may for example be detected by a detector without any focusing of the mixed beam onto the detector, by detecting the mixed output beam as a beam focused onto a detector, or by launching the mixed output beam into a single mode or multi-mode optical fiber and detecting a portion of the mixed output beam that is transmitted by the optical fiber. The effect of the beam shear and wavefront errors will also depend on properties of a beam stop should a beam stop be used in the procedure to detect the mixed output beam. Generally, the errors in the interferometric signal are compounded when an optical fiber is used to transmit the mixed output beam to the detector.
Amplitude variability of the measured interference signal can be the net result of a number of mechanisms. One mechanism is a relative beam shear of the reference and measurement components of the output beam that is for example a consequence of a change in orientation of the measurement object.
In dispersion measuring applications, optical path length measurements are made at multiple wavelengths, e.g., 532 nm and 1064 nm, and are used to measure dispersion of a gas in the measurement path of the distance measuring interferometer. The dispersion measurement can be used in converting the optical path length measured by a distance measuring interferometer into a physical length. Such a conversion can be important since changes in the measured optical path length can be caused by gas turbulence and/or by a change in the average density of the gas in the measurement arm even though the physical distance to the measurement object is unchanged.
In general, in one aspect, the invention is directed towards an interferometry system that includes a multiple-pass interferometer having reflectors to reflect at least two beams along multiple passes through the interferometer, the multiple passes including a first set of passes and a second set of passes, the reflectors having first alignments that are normal to the directions of the paths of the beams that are reflected by the reflectors, the paths of the beams being sheared during the first set of passes and during the second set of passes if at least one of the reflectors has an alignment other than the first alignment, and optics to redirect the beams after the first set of passes and before the second set of passes so that shear imparted during the second set of passes cancels shear imparted during the first set of passes.
Implementations of the invention may include one or more of the following features. The optics are configured to redirect the beams while maintaining the magnitude and direction of shear between or among the beams. The propagation path of one of the beams after being redirected by the optics is parallel to the propagation path of the beam after completing the first set of passes. The reflectors comprise plane reflection surfaces. The beams include a reference beam that is directed toward one of the reflectors maintained at a position that is stationary relative to the interferometer. The beams include a measurement beam that is directed towards one of the reflectors that is movable relative to the interferometer.
Implementations of the invention may further include one or more of the following features. The paths of the reference and measurement beams define an optical path length difference, the changes in the optical path length difference indicative of changes in the position of the one of the reflectors that is movable relative to the interferometer. The reflectors include a first reflector and a second reflector, the beams including a first beam directed toward the first reflector and a second beam directed toward the second reflector, each of the first and second reflectors being movable relative to the interferometer. The paths of the first and second beams define an optical path length difference, the changes in the optical path length difference indicative of changes in relative positions of the first and second reflectors.
Implementations of the invention may further include one or more of the following features. The first set of passes consists of two passes, and during each pass each of the beams is reflected by one of the reflectors at least once. The second set of passes consists of two passes, and during each pass each of the beams is reflected by one of the reflectors at least once. The multiple-pass interferometer includes a beam splitter that separates an input beam into the beams and directs the beams toward the reflectors. The beam splitter includes a polarizing beam splitter. The optics consist of one reflection surface or include an odd number of reflection surfaces. For each beam redirected by the optics, the beam is reflected by the plane mirrors such that a sum of angles between incident and reflection beams of the plane mirrors is zero or an integer multiple of 360 degrees, the angle measured in a direction from the incident beam to the reflection beam, the angle having a positive value when measured in a counter clockwise direction and a negative value when measured in a clockwise direction.
Implementations of the invention may further include one or more of the following features. The interferometer combines the beams after the beams travel through the first and second set of passes to form overlapping beams that exit the interferometer. The apparatus further includes a detector that responds to optical interference between the overlapping beams and produces an interference signal indicative of an optical path length difference between the paths of the beams. The detector includes a photodetector, an amplifier, and an analog-to-digital converter. The apparatus further includes an analyzer coupled to the detector to estimate a change in an optical path length difference of the beams based on the interference signal. The apparatus further includes a source to provide the beams. The two beams have different frequencies.
In general, in another aspect, the invention is directed towards a lithography system for use in fabricating integrated circuits on a wafer. The system includes a stage to support the wafer, an illumination system to image spatially patterned radiation onto the wafer, a positioning system to adjust the position of the stage relative to the imaged radiation, and at least one of the interferometry system described above. The interferometry system measures the position of the stage.
In general, in another aspect, the invention is directed towards a lithography system for use in fabricating integrated circuits on a wafer. The system includes a stage to support the wafer, an illumination system including a radiation source, a mask, a positioning system, a lens assembly, and at least one of the interferometry system described above. During operation, the source directs radiation through the mask to produce spatially patterned radiation, the positioning system adjusts the position of the mask relative to the wafer, the lens assembly images the spatially patterned radiation onto the wafer, and the interferometry system measures the position of the mask relative to the wafer.
In general, in another aspect, the invention is directed towards a beam writing system for use in fabricating a lithography mask, the system includes a source to provide a write beam to pattern a substrate, a stage to support the substrate, a beam directing assembly to deliver the write beam to the substrate, a positioning system to position the stage and beam directing assembly relative one another, and at least one of the interferometry system described above. The interferometry system measures the position of the stage relative to the beam directing assembly.
In general, in one aspect, the invention is directed towards an interferometry system that includes a multiple-pass interferometer including reflectors to reflect at least a first beam along a first path and a second beam along a second path, the first and second paths each including at least a first set of passes and a second set of passes through the interferometer, the reflectors having first alignments that are normal to the directions of the paths of the beams that are reflected by the reflectors. The relative shear between the paths of the beams changes as the beams make the first and second set of passes through the interferometer when at least one of the reflectors has an alignment other than the first alignment. The interferometry system further includes optics to redirect the beams after the first set of passes and before the second set of passes so that shear imparted during the second set of passes cancels shear imparted during the first set of passes.
Implementations of the invention may include one or more of the following features. The first and second paths do not overlap during the first and second set of passes. The interferometry system further includes a beam splitter to separate an input beam into the first and second beams prior to either of the first and second beams propagate through the first set of passes. The interferometry system further includes a second beam splitter to combine the first and second beams after both of the first and second beams propagate through the second set of passes. The interferometry system further includes a beam splitter that cooperates with the reflectors to reflect the first beam along the first path and the second beam along the second path. One of the reflectors is disposed between the beam splitter and another one of the reflectors. The multi-pass interferometer includes a differential plane mirror interferometer.
In general, in one aspect, the invention is directed towards an interferometry system that includes a multiple-pass interferometer including reflectors to reflect at least two beams along multiple passes through the interferometer, the multiple passes including a first set of passes and a second set of passes. The reflectors have first alignments. The shear between the paths of the two beams changes during the first set of passes and during the second set of passes if one of the reflectors moves from the first alignment to a second alignment different from the first alignment. The interferometry system further includes optics to redirect the beams after the first set of passes and before the second set of passes so that shear imparted during the second set of passes due to a deviation of one of the reflectors from the first alignment cancels shear imparted during the first set of passes due to the deviation.
Implementations of the invention may include one or more of the following features. The interferometer further includes a polarizing beam splitter to separate an input beam into the at least two beams. The optics comprise an odd number of plane reflection surfaces.
In general, in another aspect, the invention is directed towards an interferometry method that includes directing at least two beams along multiple passes through an interferometer, the multiple passes including a first set of passes and a second set of passes, the reflectors having first alignments that are normal to the directions of the paths of the beams that are reflected by the reflectors, and causing shear that is imparted in the first set of passes to be cancelled by shear imparted in the second set of passes by redirecting the beams after the first set of passes and before the second set of passes.
Implementations of the invention may include one or more of the following features. Redirecting the beams includes using an odd number of plane mirrors to redirect the beams. Redirecting the beams includes redirecting the beams so that the beams after being redirected travel in directions opposite but parallel to the propagation directions of the beams before being redirected. Redirecting the beams includes redirecting the beams so that the magnitude and direction of shear of the beams after being redirected are the same as the magnitude and direction of shear of the beams before being redirected. The interferometry method further includes separating an input beam into the at least two beams. The interferometry method further includes combining the beams after the multiple passes through the interferometer to form overlapping beams. The interferometry method further includes detecting interference signals from the overlapping beams. The interferometry method further includes estimating a change in the optical path length of one of the beams based on the interference signal. The interferometry method further includes estimating a change in the optical path length difference between two of the at least two beams based on the interference signal.
In general, in another aspect, the invention is directed towards a lithography method that includes supporting a wafer on a stage, imaging spatially patterned radiation onto the wafer, adjusting the position of the stage relative to the imaged radiation, and using the interferometry method described above to measure the relative position of the stage.
In general, in another aspect, the invention is directed towards a lithography method that includes supporting a wafer on a stage, directing radiation from a source through a mask to produce spatially patterned radiation, positioning the mask relative to the wafer, using the interferometry method described above to measure the position of the mask relative to the wafer, and imaging the spatially patterned radiation onto the wafer.
In general, in another aspect, the invention is directed towards a beam writing method that includes providing a write beam to pattern a substrate, supporting the substrate on a stage, delivering the write beam to the substrate, positioning the stage relative to the write beam, and using the interferometry method described above to measure the relative position of the stage.
In general, in another aspect, the invention is directed towards an interferometry method that includes directing at least two beams along multiple passes through an interferometer, the multiple passes including a first set of passes and a second set of passes, the reflectors having first alignments, and redirecting the two beams after the first set of passes and before the second set of passes to cause shear that is imparted in the first set of passes caused by one of the reflectors moving from the first alignment to a second alignment to be cancelled by shear imparted in the second set of passes caused by the movement of the reflector from the first alignment to the second alignment.
Implementations of the invention may include one or more of the following features. The interferometry method further includes separating an input beam into the at least two beams. The interferometry method further includes overlapping the two beams after the second set of passes.
Other features, objects, and advantages of the invention will be apparent from the following detailed description.